1. Field of the Invention
This invention relates to control means for the supply of operating mediums and to sheet feeding machines having the same.
2. Prior Art
A machine for feeding sheets from a stack one at a time usually has two or more suckers for lifting one edge of the uppermost sheet and also means for creating a blast of air adjacent said edge of the uppermost sheet to assist its separation from the sheet underneath. In the case of a machine employing back edge separation it is normal to provide further suckers to which the sheet to be fed is transferred after its back edge has been lifted. The further suckers then move forwardly carrying the sheet with them before releasing the sheet to continue its forward movement. The forwarding suckers then return for the next sheet which, in the meantime, has been raised at its back edge by the lifting suckers. As in the case of the lifting suckers, the forward movement of each sheet effected by the forwarding suckers is assisted by a blast of air created beneath the sheet adjacent its raised back edge. The vacuum supply to each set of suckers and both the separating air blast and the forwarding air blast are controlled and timed by separate control means or by an expensive spool valve having a separate set of ports for each of the four different functions. Such machines are well known and capable of operating at speeds of, for example, 13000 sheets/hour.
A slower type of sheet feeding machine employs front edge separation. In this case the lifting suckers are mounted for pivotal movement about a horizontal axis so that they may act as both lifting suckers and forwarding suckers. The separating air blast is provided as before but the forwarding air blast may be omitted. The control means for the vacuum to the lifting and forwarding suckers and for the separating air blast are generally the same as described above in relation to a sheet feeding machine employing back edge separation. In each case the control means suffer from the disadvantage of separate valves whose timed sequence of operation is difficult to maintain and which, up to now, was generally overcome by providing an expensive combination spool valve.